A wearyingly discursive if often very funny fictional autobiography in the form of a diary kept by the less accomplished of a proud progressive couple's deeply divergent twin sons. William, the elder (by seven minutes), grows up in the 1960s in Cambridge, Mass., among a ``distracted, quasi-Bohemian family'' that includes his cheerful, universally admired prodigy brother Clive, and parents who lovingly encourage healthful nudity and candid family discussions of adolescent masturbation and other (preferably, William would say) guilty secrets. William, who demonstrates precocity only as a curmudgeon, is in fact his exemplary brother's intellectual equal (perhaps superior). But he delights in disappointing others' expectations (for reasons never really made clear). In a skimpy chronological narrative that's nevertheless buoyed by Anastas's gift for offbeat comic phrasing, we learn of William's successive disgraces as a slow learner of the basics of infancy and childhood (in the matter of toilet training, for example, he grunts and strains inefficiently in contrast to Clive, ``a Green Beret in diapers''); a sickly adolescent who pines for beauteous schoolmate Faith Crick (Clive's girlfriend, naturally) but is usually paired with elephantine Dorothea Zimmerman; a private-school failure at ``bootcamp-style intramural sports'' and a sexual misfit who rejects his brother's offers to help him lose his virginity . . . on and on it goes, through quickly sketched college experiences, when William has a drinking problem and a girlfriend ``prone to short disappearances, broken dates and lapses of memory that included my name.'' Inconclusive further adventures in San Francisco and New York City precede the abandonment of William's diary (another task he can't complete). One wants to keep quoting, but not necessarily to keep reading what's essentially a (pretty good) one-joke book. This is, however, on balance a first novel in need of a plot.